ABSTRACT
Property characterization of nanouids has been reviewed extensively by Fan and Wang (2011), Kleinstreuer and Feng (2011), and Taylor et al. (2013). e increase in thermal conductivity depends on particle volume fraction, morphology (e.g., primary size, shape, agglomeration in the form of clusters and aggregates), dispersion, colloidal stability, and uid temperature. Large divergence exists in reported thermal conductivity enhancement in the literature and no agreement has been achieved on enhancement mechanisms. As stated by Wu, Feng et al. (2014) based on the classic eective medium theory, there is still a lot of room for thermal conductivity enhancement of nanouids. Further thermal conductivity enhancement can be obtained by manipulating the nanostructure morphology in nanouids to form elongated and/or percolated aggregates. Rheology behavior of nanouids is dierent from that of base uids. e increase in viscosity of the nanouid requires larger pumping cost and thus neutralizes the benet from the thermal conductivity enhancement. Besides, nanouids of relatively large particle volume fractions or carbon nanotube nanouids present non-Newtonian behavior, which may cause problems for heat exchangers designed for Newtonian uids. erefore, understanding of nanouid properties is critical for heat transfer applications. More investigations should be conducted to achieve large thermal conductivity enhancement with relatively low viscosity increase, for example, by modifying the nanostructure morphology in nanouids.
